go/types: types.Error.go116end end position derived from placeholder Ident.Name is garbage #81876

package p

type I interface{ M() }

var m map[int]int

func _() {
	var a int
	var b I
	a, b = m[0] // error: cannot use ok value of (comma, ok) expression ... as I value
	_, _ = a, b
}

package main

import (
	"fmt"
	"go/ast"
	"go/parser"
	"go/token"
	"go/types"
	"reflect"
)

func main() {
	for _, src := range []string{
		`package p; type I interface{ M() }; var m map[int]int; func _() { var a int; var b I; a, b = m[0]; _, _ = a, b }`,
		`package p; type I interface{ M() }; func f() (int, int); func _() { var a I; var b int; a, b = f(); _, _ = a, b }`,
	} {
		fset := token.NewFileSet()
		f, _ := parser.ParseFile(fset, "a.go", src, 0)
		conf := types.Config{Error: func(err error) {
			e := err.(types.Error)
			v := reflect.ValueOf(e)
			start, end := v.FieldByName("go116start").Int(), v.FieldByName("go116end").Int()
			fmt.Printf("%s\n\tspan [%d,%d) len=%d; file size %d\n", e.Msg, start, end, end-start, len(src))
		}}
		conf.Check("p", fset, []*ast.File{f}, nil)
	}
}